geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...

ultrahigh-temperature metamorphism (UHT) is extreme crustal

metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...

with metamorphic temperatures exceeding 900 °C.Brown, M., 2007, Metamorphic conditions in orogenic belts: a record of secular change. International Geology Review 49, 193-234Santosh, M., Omori, S., 2008a, CO₂ flushing: a plate tectonic perspective. Gondwana Research 13, 86-102 Granulite-

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

rocks metamorphosed at very high temperatures were identified in the early 1980s, although it took another decade for the

geoscience Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical, chemical, and biological complex constitutions and synergistic linkages of Earth's four spheres ...

community to recognize UHT metamorphism as a common regional phenomenon.

Petrological Petrology () is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous and metamorphic petrology are commonly taught together ...

evidence based on characteristic

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...

assemblages backed by experimental and

thermodynamic Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of ther ...

relations demonstrated that Earth's crust can attain and withstand very high temperatures (900–1000 °C) with or without partial melting.

Definition

Metamorphism of crustal rocks in which peak temperature exceeds 900 °C, recognized either by robust thermobarometry or by the presence of a diagnostic mineral assemblage in an appropriate bulk composition and oxidation state, such as assemblages with

orthopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

+ sillimanite +

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

sapphirine Sapphirine is a rare mineral, a silicate of magnesium and aluminium with the chemical formula ( (with iron as a major impurity). Named for its sapphire-like colour, sapphirine is primarily of interest to researchers and collectors: well-formed c ...

+ quartz or

spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...

+ quartz, generally at pressure conditions of sillimanite stability in meta pelites fter Brown (2007) following proposal by Harley (1998)

Identification

Petrological indicators of UHT metamorphism are usually preserved in extremely Mg-Al-rich rocks which are usually dry and restitic in nature. Mineral assemblages such as sapphirine + quartz, orthopyroxene + sillimanite ± quartz,

osumilite Osumilite is a very rare potassium-sodium-iron-magnesium-aluminium silicate mineral. Osumilite is part of the milarite group (also known as the milarite-osumilite group) of cyclosilicates. Characteristics Osumilite chemical formula is .Don S. G ...

and spinel + quartz provide straight away evidence for such extreme conditions. Occasionally widespread assemblages like

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

+ orthopyroxene, ternary

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...

s, (F-Ti)

pargasite Pargasite is a complex inosilicate mineral of the amphibole group with formula NaCa2(Mg4Al)(Si6Al2)O22(OH)2. It was first described for an occurrence in Pargas, Finland in 1814 and named for the locality. It occurs in high temperature regional ...

or metamorphic inverted pigeonite are taken as typical indicators of UHT metamorphism.

Global distribution

UHT rocks are now identified in all major continents and span different geological ages ranging from c. 3178 to 35 million years associated with major geological events. More than 46 localities/

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...

s with diagnostic UHT indicators have been reported over the globe, related to both extensional and collisional

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...

environments; the two fundamental types of Earth orogenic systems.Kelsey, D.E., 2008, On ultrahigh-temperature crustal metamorphism. Gondwana Research 13, 1-29Santosh, M., Omori, S., 2008b, CO₂ windows from mantle to atmosphere: Models on ultrahigh- temperature metamorphism and speculations on the link with melting of snowball Earth. Gondwana Research 14, in press, The major

Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth Earth ...

UHT rocks are distributed in East-Antarctica, South Africa, Russia and Canada. Paleoproterozoic UHT granulites were reported from the

North China Craton The North China Craton is a continental crustal block with one of Earth's most complete and complex records of igneous, sedimentary and metamorphic processes. It is located in northeast China, Inner Mongolia, the Yellow Sea, and North Korea. The ...

(during the

accretion Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...

of the supercontinent

Columbia Columbia may refer to: * Columbia (personification), the historical female national personification of the United States, and a poetic name for America Places North America Natural features * Columbia Plateau, a geologic and geographic region in ...

),Santosh, M. Sajeev K. and J. Li 2006, Extreme crustal metamorphism during Columbia supercontinent assembly: Evidence from North China Craton. Gondwana Research, v. 10, p. 256-266.Santosh, M., Tsunogae, T., Li, J.H., and Liu, S.J., 2007, Discovery of sapphirine- bearing Mg-Al granulites in the North China Craton: Implications for Paleoproterozoic ultrahigh- temperature metamorphism. Gondwana Research 11, 263-285.Santosh, M., Tsunogae, T., Ohyama, H. Sato, K., Li, J.H., and Liu, S.J., 2008, Carbonic metamorphism at ultrahigh-temperatures. Earth and Planetary Science Letters 266, 149-165. Taltson magmatic zone, northwestern Canada and South Harris,

Lewisian complex The Lewisian complex or Lewisian gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane and the North Atlantic Craton. These rocks are of Archaean and Paleoprote ...

, Scotland. UHT rocks from the Neoproterozoic Grenville orogeny are distributed in the Eastern Ghats Province, India. Neoproterozoic-Cambrian (Pan-African) UHT occurrences are mainly distributed in Lutzow-Holm Bay, East Antarctica, southern Madagascar, Sri Lanka and southern India. UHT rocks are also reported from younger terranes like the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

Kontum Massif, Vietnam,

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

Higo belt, Japan and

Paleogene The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...

Gruf Complex, central

Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...

. Three-million-year-old

xenoliths A xenolith ("foreign rock") is a rock fragment (country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in ign ...

erupted in Qiangtang imply that UHT metamorphism is ongoing beneath central

Tibet Tibet (; ''Böd''; ) is a region in East Asia, covering much of the Tibetan Plateau and spanning about . It is the traditional homeland of the Tibetan people. Also resident on the plateau are some other ethnic groups such as Monpa people, ...

Recent hypothesis

A correlation was proposed between the episodic formation of UHT metamorphic rocks and the episodic assembly of supercontinents in the Precambrian. However, inspection of extreme metamorphism at convergent plate margins indicates that supercontinental assembly is associated with regional HP to UHP eclogite-facies metamorphism at low thermal gradients of less than 10 °C/km, whereas continental rifting plays a crucial role in causing regional HT to UHT granulite-facies metamorphism at high thermal gradients of greater than 30 °C/km. In this regard, the episodic formation of HT to UHT granulite-facies metamorphic rocks is temporally and spatially coupled with the breakup or attempting rupture of supercontinents in the

plate tectonic Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

s context. Because UHT rocks are generally characterized by low water contents, this led to an illusion for the involvement of CO₂-rich fluids in generating diagnostic UHT assemblages according to the finding of abundant pure CO₂ fluid inclusions in these rocks. However, the extraction of liquid phases such as aqueous solutions and hydrous melts from anatectic systems during UHT metamorphism is so efficient that the common occurrence of pure CO₂ fluid inclusions looks as if the incoming CO₂ could have buffered the water activity and stabilized the

anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...

mineralogy of UHT rocks. Anatectic melts were variably extracted from anatectic systems, leading to granulite-migmatite-granite associations in accretionary and collisional orogens.Zheng, Y.-F., Chen, R.-X., 2017. Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins. Journal of Asian Earth Sciences, v. 145, p. 46-73. Metamorphic core complexes were emplaced due to the buoyant entrainment of granitic melts. The abundant water was liberated by heating dehydration of the lowest orogenic crust, contributing aqueous solutions to amphibolite-facies retrogression of the overlying crust.

Definition

Identification

Global distribution

Recent hypothesis

References

Further reading